Thanks once again to all for your help. I flushed out the heat exchanger and found that the hot water outlet to the tank was blocked solid. I have temp. fixed the problem and everything is now working fine, but i feel the problem will return, so i am going to have the hot water cylinder replaced. It looks like i have a primatic system (as described by "croydon corgi").
Your assistance with this problem has been very much appreciated.
Aljack
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